1. Field of the Invention
This invention relates to a system for and method of pacing a malfunctioning heart and, more particularly, to such a system and method which effects pacing in response to sensing hemodynamic change at a site in the circulatory system of a patient. The invention provides for pacing of a malfunctioning heart and, more particularly, to varying the escape interval or rate of a pacemaker in response to a hemodynamic parameter.
2. Description of the Prior Art
In recent years, substantial progress has been made in pacemakers for treating various heart disorders and arrhythmias. Past efforts have resulted in the development of implantable electronic pacemakers which, in response to the detection of an abnormal cardiac condition, effect a restoration of cardiac rhythm by pacing.
External and implantable pacemakers for the therapeutic stimulation of the heart are well known in the art. Initially these cardiac pacers were asynchronous in operation providing stimulating pulses to the heart at a fixed rate independent of the physiological demand of the patient. Later demand pacemakers were developed as exemplified by U.S. Pat. No. 3,345,990 to Berkovitz. A further example of an early demand pacemaker is disclosed in U.S. Pat. No. 3,478,746 to Greatback. These disclosed devices provide stimulating pulses to the heart only in the absence of naturally occurring cardiac activity. This known form of pacing system permits the underlying cardiac rhythm of a patient to inhibit the pacemaker if the patient's intrinsic heart rate is above the preset escape interval of the pacing system. However, if the intrinsic cardiac activity of the patient drops below the minimum rate set by the escape interval of the pacing system, stimulating pulses will be supplied to the heart. In this fashion the demand pacemaker provides a lower boundary rate below which the patient's heart will not be permitted to drop.
Subsequently, more effective demand pacemakers of the hysteresis type were developed, an example being disclosed in the U.S. Pat. No. RE 28,003 to Gobeli. This hysteresis pacemaker permits the heart to inhibit the pacemaker down to a sentinel rate set by the hysteresis pacemaker. However, if no intrinsic cardiac activity is detected during the sentinel escape interval, the patient's heart will be stimulated at a nominal escape interval which is somewhat shorter than the lower hysteresis rate. In operation, the hysteresis pacemaker alters the escape interval in response to detected cardiac events.
More recently, pacemakers have been disclosed which rely upon a historical average of detected cardiac activity to set the escape interval. An example of one such pacer is disclosed in U.S. Pat. No. 3,921,642 to Preston.
Other forms of rate-adaptive pacemakers have also been proposed. These pacemakers rely on the sensing of atrial activity, blood pH, respiratory rate or QT interval data to alter the escape interval of the pacemaker. Discussions of some of these prior art proposals may be found in "Relation Between the QT Interval and Heart Rate", Rickards et al., Britt Heart J., 1981; 45; 56-61 and in "A Physiologically Controlled Cardiac Pacemaker", Krasner, Voukydis and Nardella, J.A.A.M.I, Volume I, No. 3, pgs. 14-20, 1966.
Recently, a pacemaker which alters the escape interval in response to the physiological demand or needs of the patient has been disclosed in U.S. Pat. No. 4,428,378 to Anderson et al. in which an activity sensor, mounted within an implanted pacemaker, detects the activity level of the patient and alters the escape interval between a preset minimum and maximum in response to the sensed activity level.
None of the above-mentioned prior art pacemakers involve the direct sensing of a hemodynamic parameter nor the concept of automatically adjusting the escape interval or rate of a pacemaker in response to changes in the sensed hemodynamic parameter.
A theoretical right atrial pressure feedback system to restore control of a rate-limited heart has been proposed in the publication of Todd J. Cohen, entitled "A Theoretical Right Atrial Pressure Feedback Heart Rate Control System to Restore Physiologic Control to a Rate-Limited Heart", Pace, Vol. 7, pgs. 671-677, July-August 1984. The disclosed system involves comparing a signal corresponding to right atrial pressure, as sensed by a feedback sensor, with a fixed signal corresponding to a command right atrial pressure. The comparison develops an error signal which, after processing, is used to control the impulse frequency of a pacemaker.
An implantable pacemaker pacing a heart in accordance with the heart rate needed to produce a required cardiac output in which a sensor is used to sense right ventricular systolic pressure is disclosed in the U.S. Pat. No. 4,566,456 to Koning et al. A microprocessor is provided to relate right ventricular pressure and/or the time derivative thereof with the rate needed to produce a desired cardiac output and to cause the pacemaker to pace the heart and, thus, produce the desired output when the heart is not naturally paced.
Neither the theoretical system of Cohen, supra, nor the system disclosed in the patent of Koning et al., supra, involves the sensing of a long term baseline pressure and its comparison with a short term current pressure to develop a signal for varying either the rate or escape interval of a pacemaker.